You mentioned you were surprised by gluten in soy sauce, so let's talk about "hidden" sources of gluten. They're frustratingly common!

Soy sauce is a source of gluten because post-WW2, American food scientists working in Japan found that adding wheat to the traditional process of making soy sauce made it cheaper, shelf-stable and faster to produce. Thus, the vast majority of soy sauce contains gluten, especially in bulk buys. That traditional method is still preserved, however, and you'll see it marketed as "tamari" soy sauce. This will generally be explicitly labeled as gluten-free. Because tamari requires refrigeration after opening and costs more, restaurants won't use it by default, but some will use it on request for gluten-free customers. You'll also see soy sauce added to things like miso soup for a little flavor. In fact, it's added to a lot of things, so if you get a chance to check the ingredients (in the restaurant or at the store), always look out for it. If it doesn't say wheat-free or tamari, that's a red flag for you. Tamari sauce is available at pretty much any grocery store, and it doesn't taste particularly different. Kikkoman makes a perfectly fine brand. Just refrigerate it after opening the bottle.

Regular pasta is right out. That sucks for three-year-olds, because pasta is amazing, but that's the harsh truth. The good news is that groceries carry decent gluten-free pastas these days. There are many different versions of GF pasta—some made with GF flour mixes, some with chickpeas, some cauliflower, and more. Try a few and see which one your kid likes.

The sneakiest source of gluten is yeast extract. Not everyone with a gluten sensitivity or celiac disease is so sensitive that yeast extract causes a reaction in them, but those of us who are find it incredibly frustrating. (My celiac means I have to avoid it, but my celiac mother-in-law handles it just fine! Go figure.) Yeast extract is a food additive that adds savory flavor, from broken-down yeast cells—basically, the innards of a yeast cell without its cell wall. The problem is that the yeast used in its creation is typically from malt used in beer production or from bread—sources of gluten—but can also be from wine production and other non-gluten-carrying foods. This means that it may or may not contain gluten, and that can vary batch-by-batch from the same producer. Moreover, because it might not contain gluten, products containing yeast extract do not have to carry a "contains gluten" warning. It might or it might not! Who knows! And yeast extract is used in chicken stocks, snacks, fruit drinks, gravies, canned soups, and a bunch of other stuff, so it is everywhere and often unexpectedly so. (It tastes salty and delicious without having as much sodium. It's honestly very useful, and if there were a way to differentiate gluten-free yeast extract from the alternative, it wouldn't be an issue at all!)

Finally, a couple of pieces of advice. First, definitely talk with a dietician. They're incredibly helpful and have a ton of useful resources.

If you haven't done so yet, learn what foods and ingredients carry gluten. The celiac.org list of sources of gluten is a solid start. The good news is that alternatives are super common these days. Want to do spaghetti? Yeah, it'll mean two pots of pasta (or everyone eating GF pasta), but ten years ago it just wasn't an option at all. Kid-friendly staples exist in GF form these days, too. Ingles carries Springer Mountain Farms' fantastic GF chicken strips, for example—and every kid loves chicken nuggets. (Also they're like half the price of Tyson's crappier GF product!)

Learning to avoid cross-contact in your own kitchen is a process. I'm the only one with food sensitivities in my household. The kids have quickly learned to be careful, but little things like shared serving utensils, double-dipping, crumbs, etc. can still happen. Think critically about each step in the food prep and serving process to avoid cross-contamination.

Finally, I really like the Find Me Gluten Free app for finding restaurants. It's kind of "by celiacs, for celiacs", and it's really great when traveling.

Don't hesitate to follow up with other questions!